It's a fairly long, but mostly hidden process requiring a crouch jump.
The first two jumps are fairly obvious. The ledge 3 is a bit less obvious; it looks like decoration, but it's actually a platform you can land on. It's even less obvious that this ledge actually expands in the area marked 4. Once you are against the 'protection', check the leaderboard for respawn vaves, and crouch jump (tap jump and crouch at the same time). Sap to your heart's desire.
Here's a video demonstration, too.
There used to be an exploitable oversight right here, as this particular spawn door was not protected and enemies could go through while it was open (but not open it themselves). This was fixed in the July 7, 2011 Patch.